Five Russian nationals from the Chechen autonomous republic were detained overnight, four of them at Saint-Jean-de-Védas, near Montpéllier, and the other in Béziers, prosecutor Yvon Calvet told the Midi Libre.
Explosives were seized in Béziers, the regional newspaper reported, but it was not yet certain that an attack was planned.
A court in Haskovo, Bulgaria, on Tuesday ruled that Muslim convert, Fritz-Joly Joachin, 29, could be extradited in response to a French arrest warrant.
Joachin, who is of Haitian origin, admits knowing the Kouachi brothers but denies being a radical Islamist.
“I am innocent. I want to return to France,” he told the court.
He was arrested on 1 January as he tried to cross from Bulgaria to Turkey and admitted being “old friends” with the Kouachi.
Last week he said he used to play football with the brothers and had a "business connection" selling clothes but denied knowing about their plans to launch an Islamist attack.
According to the French arrest warrant issued, Joachin was travelling on a bus with three other people who knew the Kouachis when he was arrested.
The other three were allowed to continue travelling to Turkey.